A hot-dip galvannealed steel sheet (hereinafter also briefly referred to as “GA steel sheet”) is obtained by heating a hot-dip galvanized steel sheet (GI steel sheet) to allow iron in the base steel sheet to diffuse into a galvanized layer to thereby alloy iron and zinc (Zn). Such GA steel sheets excel typically in strength, weldability, and corrosion resistance after coating and are used typically as steel sheets for automobile bodies.
The GA steel sheets, when used for the above usage, are subjected to coating (painting), and, before coating, they are generally subjected to phosphating as a surface treatment for coating. It is important to deposit a satisfactory phosphate crystal coating as a result of the phosphating, for ensuring satisfactory coating properties such as coating adhesion and corrosion resistance.
GA steel sheets as intact are known to exhibit superior phosphatability. This is because the surface of the galvanized layer is composed of a Zn—Fe alloy having satisfactory reactivity with a phosphating agent and contains substantially no impurities.
On the other hand, high-tensile (high-strength) steel sheets have been widely used in automobile industries, in order to improve collision safety and to increase fuel efficiency as a result of weight reduction. For providing steel sheets with higher tensile, reinforcing elements such as Si, Al, Mn, P, Cr, Mo, and Ti are incorporated into base steel sheets. However, when a steel sheet containing these elements is used as a base steel sheet and subjected to hot-dip galvanizing and alloying (galvannealing), the respective elements diffuse with iron into a galvanized layer during alloying process after galvanization and are contained as impurities in the galvanized layer. The resulting GA steel sheet suffers from instable phosphatability due to the added elements contained during galvanization, although such a GA steel sheet, if not containing these elements, exhibits satisfactory phosphatability.
In this connection, Si and Mn are mainly used as reinforcing elements for the production of a high-tensile steel sheet. Upon galvanization of the surface of a steel sheet containing these elements, an effective method for preventing generation of bare spots and for stably ensuring satisfactory appearance quality is a method of oxidizing the surface of the steel sheet, carrying out annealing in a hydrogen-containing atmosphere (reduction annealing), and subsequently carrying out galvanization (hereinafter this method is also referred to as “oxidation-reduction galvanizing method”) (for example, Patent Document 1).
In the oxidation-reduction galvanizing method, Si and Mn in the steel sheet are oxidized to form oxides simultaneously with the oxidization of iron during the oxidation process; but Si and Mn remain as oxides without being reduced in the subsequent reduction process, although iron is reduced in this process. The remained oxides are contaminated and dispersed with iron into a galvanized layer in the subsequent galvanizing/alloying process. Depending on the oxidation conditions, the magnitudes of the generations of silicon oxides and manganese oxides vary, and the amounts of these oxides dispersed into the galvanized layer also vary.
There are disclosed techniques relating to GA steel sheets containing oxides in the galvanized layer (Patent Documents 2 and 3). These techniques, however, fail to teach about the amounts of oxides in the galvanized layer, although they mention the presence of the oxides. The GA steel sheets disclosed in these documents are produced by carrying out acid pickling of the base steel sheet under controlled conditions before galvanization, and adjusting the partial pressures of water vapor and hydrogen in a reduction furnace, and this technique is fundamentally different from the oxidation-reduction galvanizing method. Additionally, these techniques are intended to improve deposit adhesion and alloying processability, respectively, but do not pay attention to phosphatability. Specifically, techniques for improving the phosphatability of a GA steel sheet containing oxides in its galvanized layer have not yet been established.
Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. 122865/1980
Patent Document 2: JP-A No. 204280/2004
Patent Document 3: JP-A No. 315960/2004